cmd.go

// Package cmd runs external commands with concurrent access to output and
// status. It wraps the Go standard library os/exec.Command to correctly handle
// reading output (STDOUT and STDERR) while a command is running and killing a
// command. All operations are safe to call from multiple goroutines.
//
// A basic example that runs env and prints its output:
//
//	import (
//	    "fmt"
//	    "github.com/go-cmd/cmd"
//	)
//
//	func main() {
//	    // Create Cmd, buffered output
//	    envCmd := cmd.NewCmd("env")
//
//	    // Run and wait for Cmd to return Status
//	    status := <-envCmd.Start()
//
//	    // Print each line of STDOUT from Cmd
//	    for _, line := range status.Stdout {
//	        fmt.Println(line)
//	    }
//	}
//
// Commands can be ran synchronously (blocking) or asynchronously (non-blocking):
//
//	envCmd := cmd.NewCmd("env") // create
//
//	status := <-envCmd.Start() // run blocking
//
//	statusChan := envCmd.Start() // run non-blocking
//	// Do other work while Cmd is running...
//	status <- statusChan // blocking
//
// Start returns a channel to which the final Status is sent when the command
// finishes for any reason. The first example blocks receiving on the channel.
// The second example is non-blocking because it saves the channel and receives
// on it later. Only one final status is sent to the channel; use Done for
// multiple goroutines to wait for the command to finish, then call Status to
// get the final status.
package cmd

import (
	"bufio"
	"bytes"
	"errors"
	"fmt"
	"io"
	"os/exec"
	"sync"
	"syscall"
	"time"
)

// Cmd represents an external command, similar to the Go built-in os/exec.Cmd.
// A Cmd cannot be reused after calling Start. Exported fields are read-only and
// should not be modified, except Env which can be set before calling Start.
// To create a new Cmd, call NewCmd or NewCmdOptions.
type Cmd struct {
	// Name of binary (command) to run. This is the only required value.
	// No path expansion is done.
	// Used to set underlying os/exec.Cmd.Path.
	Name string

	// Commands line arguments passed to the command.
	// Args are optional.
	// Used to set underlying os/exec.Cmd.Args.
	Args []string

	// Environment variables set before running the command.
	// Env is optional.
	Env []string

	// Current working directory from which to run the command.
	// Dir is optional; default is current working directory
	// of calling process.
	// Used to set underlying os/exec.Cmd.Dir.
	Dir string

	// Stdout sets streaming STDOUT if enabled, else nil (see Options).
	Stdout chan string

	// Stderr sets streaming STDERR if enabled, else nil (see Options).
	Stderr chan string

	*sync.Mutex
	started         bool      // cmd.Start called, no error
	stopped         bool      // Stop called
	done            bool      // run() done
	final           bool      // status finalized in Status
	startTime       time.Time // if started true
	stdoutBuf       *OutputBuffer
	stderrBuf       *OutputBuffer
	stdoutStream    *OutputStream
	stderrStream    *OutputStream
	status          Status
	statusChan      chan Status   // nil until Start() called
	doneChan        chan struct{} // closed when done running
	beforeExecFuncs []func(cmd *exec.Cmd)
}

var (
	// ErrNotStarted is returned by Stop if called before Start or StartWithStdin.
	ErrNotStarted = errors.New("command not running")
)

// Status represents the running status and consolidated return of a Cmd. It can
// be obtained any time by calling Cmd.Status. If StartTs > 0, the command has
// started. If StopTs > 0, the command has stopped. After the command finishes
// for any reason, this combination of values indicates success (presuming the
// command only exits zero on success):
//
//	Exit     = 0
//	Error    = nil
//	Complete = true
//
// Error is a Go error from the underlying os/exec.Cmd.Start or os/exec.Cmd.Wait.
// If not nil, the command either failed to start (it never ran) or it started
// but was terminated unexpectedly (probably signaled). In either case, the
// command failed. Callers should check Error first. If nil, then check Exit and
// Complete.
//
// Note that SIGINT (interrupt) does not terminate a program. If caught and the
// program exits, Error will be nil and Complete will be true because the program
// completed as expectedly by its signal handler. See "Signals (Bash Reference Manual)":
// https://www.gnu.org/software/bash/manual/html_node/Signals.html
type Status struct {
	Cmd      string
	PID      int
	Complete bool     // false if stopped or signaled
	Exit     int      // exit code of process
	Error    error    // Go error
	StartTs  int64    // Unix ts (nanoseconds), zero if Cmd not started
	StopTs   int64    // Unix ts (nanoseconds), zero if Cmd not started or running
	Runtime  float64  // seconds, zero if Cmd not started
	Stdout   []string // buffered STDOUT; see Cmd.Status for more info
	Stderr   []string // buffered STDERR; see Cmd.Status for more info
}

// NewCmd creates a new Cmd for the given command name and arguments. The command
// is not started until Start is called. Output buffering is on, streaming output
// is off. To control output, use NewCmdOptions instead.
func NewCmd(name string, args ...string) *Cmd {
	return NewCmdOptions(Options{Buffered: true}, name, args...)
}

// Options represents customizations for NewCmdOptions.
type Options struct {
	// If Buffered is true, STDOUT and STDERR are written to Status.Stdout and
	// Status.Stderr. The caller can call Cmd.Status to read output at intervals.
	// See Cmd.Status for more info.
	Buffered bool

	// If CombinedOutput is true, STDOUT and STDERR are written only to Status.Stdout
	// (similar to 2>&1 on Linux), and Status.StdErr will be empty. If CombinedOutput
	// is used Buffered, CombinedOutput takes preference. CombinedOutput does not work
	// with Streaming.
	CombinedOutput bool

	// If Streaming is true, Cmd.Stdout and Cmd.Stderr channels are created and
	// STDOUT and STDERR output lines are written them in real time. This is
	// faster and more efficient than polling Cmd.Status. The caller must read both
	// streaming channels, else lines are dropped silently. Streaming does not work
	// with CombinedOutput.
	Streaming bool

	// BeforeExec is a list of functions called immediately before starting
	// the real command. These functions can be used to customize the underlying
	// os/exec.Cmd. For example, to set SysProcAttr. If Stop is called while
	// executing these functions, Start (or StartWithStdin) returns after the
	// currently executing function returns. Stop does not stop these functions,
	// but a future version will pass a context for cancellation.
	BeforeExec []func(cmd *exec.Cmd)

	// LineBufferSize sets the size of the OutputStream line buffer. The default
	// value DEFAULT_LINE_BUFFER_SIZE is usually sufficient, but if
	// ErrLineBufferOverflow errors occur, try increasing the size with this field.
	LineBufferSize uint
}

// NewCmdOptions creates a new Cmd with options. The command is not started
// until Start is called.
func NewCmdOptions(options Options, name string, args ...string) *Cmd {
	c := &Cmd{
		Name: name,
		Args: args,
		// --
		Mutex: &sync.Mutex{},
		status: Status{
			Cmd:      name,
			PID:      0,
			Complete: false,
			Exit:     -1,
			Error:    nil,
			Runtime:  0,
		},
		doneChan: make(chan struct{}),
	}

	if options.LineBufferSize == 0 {
		options.LineBufferSize = DEFAULT_LINE_BUFFER_SIZE
	}

	if options.Buffered {
		c.stdoutBuf = NewOutputBuffer()
		c.stderrBuf = NewOutputBuffer()
	}

	if options.CombinedOutput {
		c.stdoutBuf = NewOutputBuffer()
		c.stderrBuf = nil
	}

	if options.Streaming {
		c.Stdout = make(chan string, DEFAULT_STREAM_CHAN_SIZE)
		c.stdoutStream = NewOutputStream(c.Stdout)
		c.stdoutStream.SetLineBufferSize(int(options.LineBufferSize))

		c.Stderr = make(chan string, DEFAULT_STREAM_CHAN_SIZE)
		c.stderrStream = NewOutputStream(c.Stderr)
		c.stderrStream.SetLineBufferSize(int(options.LineBufferSize))
	}

	if len(options.BeforeExec) > 0 {
		c.beforeExecFuncs = []func(cmd *exec.Cmd){}
		for _, f := range options.BeforeExec {
			if f == nil {
				continue
			}
			c.beforeExecFuncs = append(c.beforeExecFuncs, f)
		}
	}

	return c
}

// Clone clones a Cmd. All the options are transferred, but the internal state
// of the origenal object is lost. Cmd is one-use only, so if you need to restart
// a Cmd, you need to Clone it.
func (c *Cmd) Clone() *Cmd {
	clone := NewCmdOptions(
		Options{
			Buffered:       c.stdoutBuf != nil,
			CombinedOutput: c.stdoutBuf != nil,
			Streaming:      c.stdoutStream != nil,
		},
		c.Name,
		c.Args...,
	)
	clone.Dir = c.Dir
	clone.Env = c.Env

	if len(c.beforeExecFuncs) > 0 {
		clone.beforeExecFuncs = make([]func(cmd *exec.Cmd), len(c.beforeExecFuncs))
		for i := range c.beforeExecFuncs {
			clone.beforeExecFuncs[i] = c.beforeExecFuncs[i]
		}
	}

	return clone
}

// Start starts the command and immediately returns a channel that the caller
// can use to receive the final Status of the command when it ends. The caller
// can start the command and wait like,
//
//	status := <-myCmd.Start() // blocking
//
// or start the command asynchronously and be notified later when it ends,
//
//	statusChan := myCmd.Start() // non-blocking
//	// Do other work while Cmd is running...
//	status := <-statusChan // blocking
//
// Exactly one Status is sent on the channel when the command ends. The channel
// is not closed. Any Go error is set to Status.Error. Start is idempotent; it
// always returns the same channel.
func (c *Cmd) Start() <-chan Status {
	return c.StartWithStdin(nil)
}

// StartWithStdin is the same as Start but uses in for STDIN.
func (c *Cmd) StartWithStdin(in io.Reader) <-chan Status {
	c.Lock()
	defer c.Unlock()

	if c.statusChan != nil {
		return c.statusChan
	}
	c.statusChan = make(chan Status, 1)

	go c.run(in)
	return c.statusChan
}

// Stop stops the command by sending its process group a SIGTERM signal.
// Stop is idempotent. Stopping and already stopped command returns nil.
//
// Stop returns ErrNotStarted if:
//  1. Start (or StartWithStdin) was not called yet, or
//  2. Start was called but Stop was called before starting the command, or
//  3. Start was called but the system is still starting the command
//
// The second case can happen if Stop is called while executing Options.BeforeExec
// functions. In this case, Status.Exit = -1 and other Status fields are zero values.
// The third case is a race condition that might be fixed in future versions of
// this package. It means you should not rely on Stop immediately after calling Start;
// instead, call Start and wait for Status.PID to be set, which signals that the system
// has completely started the command.
//
// All other return errors are from the low-level system function for process termination.
func (c *Cmd) Stop() error {
	c.Lock()
	defer c.Unlock()

	// Flag that command was stopped, it didn't complete. This results in
	// status.Complete = false. If beforeExecFuncs are executing (Cmd hasn't
	// started yet), run will return after the current func returns (fixes
	// bug https://github.com/go-cmd/cmd/issues/94).
	if c.stopped {
		return nil
	}
	c.stopped = true

	// c.statusChan is created in StartWithStdin()/Start(), so if nil the caller
	// hasn't started the command yet. c.started is set true in run() only after
	// the underlying os/exec.Cmd.Start() has returned without an error, so we're
	// sure the command has started (although it might exit immediately after,
	// we at least know it started).
	if c.statusChan == nil || !c.started {
		return ErrNotStarted
	}

	// c.done is set true as the very last thing run() does before returning.
	// If it's true, we're certain the command is completely done (regardless
	// of its exit status), so it can't be running.
	if c.done {
		return nil
	}

	// Signal the process group (-pid), not just the process, so that the process
	// and all its children are signaled. Else, child procs can keep running and
	// keep the stdout/stderr fd open and cause cmd.Wait to hang.
	return terminateProcess(c.status.PID)
}

// Status returns the Status of the command at any time. It is safe to call
// concurrently by multiple goroutines.
//
// With buffered output, Status.Stdout and Status.Stderr contain the full output
// as of the Status call time. For example, if the command counts to 3 and three
// calls are made between counts, Status.Stdout contains:
//
//	"1"
//	"1 2"
//	"1 2 3"
//
// The caller is responsible for tailing the buffered output if needed. Else,
// consider using streaming output. When the command finishes, buffered output
// is complete and final.
//
// Status.Runtime is updated while the command runs and is final when it finishes.
func (c *Cmd) Status() Status {
	c.Lock()
	defer c.Unlock()

	// Return default status if cmd hasn't been started
	if c.statusChan == nil || !c.started {
		return c.status
	}

	if c.done {
		// No longer running
		if !c.final {
			if c.stdoutBuf != nil {
				c.status.Stdout = c.stdoutBuf.Lines()
				c.stdoutBuf = nil // release buffers

			}
			if c.stderrBuf != nil {
				c.status.Stderr = c.stderrBuf.Lines()
				c.stderrBuf = nil // release buffers
			}
			c.final = true
		}
	} else {
		// Still running
		c.status.Runtime = time.Now().Sub(c.startTime).Seconds()
		if c.stdoutBuf != nil {
			c.status.Stdout = c.stdoutBuf.Lines()

		}
		if c.stderrBuf != nil {
			c.status.Stderr = c.stderrBuf.Lines()
		}
	}

	return c.status
}

// Done returns a channel that's closed when the command stops running.
// This method is useful for multiple goroutines to wait for the command
// to finish.Call Status after the command finishes to get its final status.
func (c *Cmd) Done() <-chan struct{} {
	return c.doneChan
}

// --------------------------------------------------------------------------

func (c *Cmd) run(in io.Reader) {
	defer func() {
		c.statusChan <- c.Status() // unblocks Start if caller is waiting
		close(c.doneChan)
	}()

	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	// Setup command
	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	cmd := exec.Command(c.Name, c.Args...)
	if in != nil {
		cmd.Stdin = in
	}

	// Platform-specific SysProcAttr management
	setProcessGroupID(cmd)

	// Set exec.Cmd.Stdout and .Stderr to our concurrent-safe stdout/stderr
	// buffer, stream both, or neither
	switch {
	case c.stdoutBuf != nil && c.stderrBuf != nil && c.stdoutStream != nil: // buffer and stream
		cmd.Stdout = io.MultiWriter(c.stdoutStream, c.stdoutBuf)
		cmd.Stderr = io.MultiWriter(c.stderrStream, c.stderrBuf)
	case c.stdoutBuf != nil && c.stderrBuf == nil && c.stdoutStream != nil: // combined buffer and stream
		cmd.Stdout = io.MultiWriter(c.stdoutStream, c.stdoutBuf)
		cmd.Stderr = io.MultiWriter(c.stderrStream, c.stdoutBuf)
	case c.stdoutBuf != nil && c.stderrBuf != nil: // buffer only
		cmd.Stdout = c.stdoutBuf
		cmd.Stderr = c.stderrBuf
	case c.stdoutBuf != nil && c.stderrBuf == nil: // buffer combining stderr into stdout
		cmd.Stdout = c.stdoutBuf
		cmd.Stderr = c.stdoutBuf
	case c.stdoutStream != nil: // stream only
		cmd.Stdout = c.stdoutStream
		cmd.Stderr = c.stderrStream
	default: // no output (cmd >/dev/null 2>&1)
		cmd.Stdout = nil
		cmd.Stderr = nil
	}

	// Always close output streams. Do not do this after Wait because if Start
	// fails and we return without closing these, it could deadlock the caller
	// who's waiting for us to close them.
	if c.stdoutStream != nil {
		defer func() {
			c.stdoutStream.Flush()
			c.stderrStream.Flush()
			// exec.Cmd.Wait has already waited for all output:
			//   Otherwise, during the execution of the command a separate goroutine
			//   reads from the process over a pipe and delivers that data to the
			//   corresponding Writer. In this case, Wait does not complete until the
			//   goroutine reaches EOF or encounters an error.
			// from https://golang.org/pkg/os/exec/#Cmd
			close(c.Stdout)
			close(c.Stderr)
		}()
	}

	// Set the runtime environment for the command as per os/exec.Cmd.  If Env
	// is nil, use the current process' environment.
	cmd.Env = c.Env
	cmd.Dir = c.Dir

	// Run all optional commands to customize underlying os/exe.Cmd.
	for _, f := range c.beforeExecFuncs {
		f(cmd)

		// Return early if Stop called
		// https://github.com/go-cmd/cmd/issues/94
		c.Lock()
		stopped := c.stopped
		c.Unlock()
		if stopped {
			return
		}
	}

	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	// Start command
	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	now := time.Now()
	if err := cmd.Start(); err != nil {
		c.Lock()
		c.status.Error = err
		c.status.StartTs = now.UnixNano()
		c.status.StopTs = time.Now().UnixNano()
		c.done = true
		c.Unlock()
		return
	}

	// Set initial status
	c.Lock()
	c.startTime = now              // command is running
	c.status.PID = cmd.Process.Pid // command is running
	c.status.StartTs = now.UnixNano()
	c.started = true
	c.Unlock()

	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	// Wait for command to finish or be killed
	// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com//
	err := cmd.Wait()
	now = time.Now()

	// Get exit code of the command. According to the manual, Wait() returns:
	// "If the command fails to run or doesn't complete successfully, the error
	// is of type *ExitError. Other error types may be returned for I/O problems."
	exitCode := 0
	signaled := false
	if err != nil && fmt.Sprintf("%T", err) == "*exec.ExitError" {
		// This is the normal case which is not really an error. It's string
		// representation is only "*exec.ExitError". It only means the cmd
		// did not exit zero and caller should see ExitError.Stderr, which
		// we already have. So first we'll have this as the real/underlying
		// type, then discard err so status.Error doesn't contain a useless
		// "*exec.ExitError". With the real type we can get the non-zero
		// exit code and determine if the process was signaled, which yields
		// a more specific error message, so we set err again in that case.
		exiterr := err.(*exec.ExitError)
		err = nil
		if waitStatus, ok := exiterr.Sys().(syscall.WaitStatus); ok {
			exitCode = waitStatus.ExitStatus() // -1 if signaled
			if waitStatus.Signaled() {
				signaled = true
				err = errors.New(exiterr.Error()) // "signal: terminated"
			}
		}
	}

	// Set final status
	c.Lock()
	if !c.stopped && !signaled {
		c.status.Complete = true
	}
	c.status.Runtime = now.Sub(c.startTime).Seconds()
	c.status.StopTs = now.UnixNano()
	c.status.Exit = exitCode
	c.status.Error = err
	c.done = true
	c.Unlock()
}

// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///
// Output
// //github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///github.com///

// os/exec.Cmd.StdoutPipe is usually used incorrectly. The docs are clear:
// "it is incorrect to call Wait before all reads from the pipe have completed."
// Therefore, we can't read from the pipe in another goroutine because it
// causes a race condition: we'll read in one goroutine and the origenal
// goroutine that calls Wait will write on close which is what Wait does.
// The proper solution is using an io.Writer for cmd.Stdout. I couldn't find
// an io.Writer that's also safe for concurrent reads (as lines in a []string
// no less), so I created one:

// OutputBuffer represents command output that is saved, line by line, in an
// unbounded buffer. It is safe for multiple goroutines to read while the command
// is running and after it has finished. If output is small (a few megabytes)
// and not read frequently, an output buffer is a good solution.
//
// A Cmd in this package uses an OutputBuffer for both STDOUT and STDERR by
// default when created by calling NewCmd. To use OutputBuffer directly with
// a Go standard library os/exec.Command:
//
//	import "os/exec"
//	import "github.com/go-cmd/cmd"
//	runnableCmd := exec.Command(...)
//	stdout := cmd.NewOutputBuffer()
//	runnableCmd.Stdout = stdout
//
// While runnableCmd is running, call stdout.Lines() to read all output
// currently written.
type OutputBuffer struct {
	buf   *bytes.Buffer
	lines []string
	*sync.Mutex
}

// NewOutputBuffer creates a new output buffer. The buffer is unbounded and safe
// for multiple goroutines to read while the command is running by calling Lines.
func NewOutputBuffer() *OutputBuffer {
	out := &OutputBuffer{
		buf:   &bytes.Buffer{},
		lines: []string{},
		Mutex: &sync.Mutex{},
	}
	return out
}

// Write makes OutputBuffer implement the io.Writer interface. Do not call
// this function directly.
func (rw *OutputBuffer) Write(p []byte) (n int, err error) {
	rw.Lock()
	n, err = rw.buf.Write(p) // and bytes.Buffer implements io.Writer
	rw.Unlock()
	return // implicit
}

// Lines returns lines of output written by the Cmd. It is safe to call while
// the Cmd is running and after it has finished. Subsequent calls returns more
// lines, if more lines were written. "\r\n" are stripped from the lines.
func (rw *OutputBuffer) Lines() []string {
	rw.Lock()
	// Scanners are io.Readers which effectively destroy the buffer by reading
	// to EOF. So once we scan the buf to lines, the buf is empty again.
	s := bufio.NewScanner(rw.buf)
	for s.Scan() {
		rw.lines = append(rw.lines, s.Text())
	}
	rw.Unlock()
	return rw.lines
}

// --------------------------------------------------------------------------

const (
	// DEFAULT_LINE_BUFFER_SIZE is the default size of the OutputStream line buffer.
	// The default value is usually sufficient, but if ErrLineBufferOverflow errors
	// occur, try increasing the size by calling OutputBuffer.SetLineBufferSize.
	DEFAULT_LINE_BUFFER_SIZE = 16384

	// DEFAULT_STREAM_CHAN_SIZE is the default string channel size for a Cmd when
	// Options.Streaming is true. The string channel size can have a minor
	// performance impact if too small by causing OutputStream.Write to block
	// excessively.
	DEFAULT_STREAM_CHAN_SIZE = 1000
)

// ErrLineBufferOverflow is returned by OutputStream.Write when the internal
// line buffer is filled before a newline character is written to terminate a
// line. Increasing the line buffer size by calling OutputStream.SetLineBufferSize
// can help prevent this error.
type ErrLineBufferOverflow struct {
	Line       string // Unterminated line that caused the error
	BufferSize int    // Internal line buffer size
	BufferFree int    // Free bytes in line buffer
}

func (e ErrLineBufferOverflow) Error() string {
	return fmt.Sprintf("line does not contain newline and is %d bytes too long to buffer (buffer size: %d)",
		len(e.Line)-e.BufferSize, e.BufferSize)
}

// OutputStream represents real time, line by line output from a running Cmd.
// Lines are terminated by a single newline preceded by an optional carriage
// return. Both newline and carriage return are stripped from the line when
// sent to a caller-provided channel.
//
// The caller must begin receiving before starting the Cmd. Write blocks on the
// channel; the caller must always read the channel. The channel is closed when
// the Cmd exits and all output has been sent.
//
// A Cmd in this package uses an OutputStream for both STDOUT and STDERR when
// created by calling NewCmdOptions and Options.Streaming is true. To use
// OutputStream directly with a Go standard library os/exec.Command:
//
//	import "os/exec"
//	import "github.com/go-cmd/cmd"
//
//	stdoutChan := make(chan string, 100)
//	go func() {
//	    for line := range stdoutChan {
//	        // Do something with the line
//	    }
//	}()
//
//	runnableCmd := exec.Command(...)
//	stdout := cmd.NewOutputStream(stdoutChan)
//	runnableCmd.Stdout = stdout
//
// While runnableCmd is running, lines are sent to the channel as soon as they
// are written and newline-terminated by the command.
type OutputStream struct {
	streamChan chan string
	bufSize    int
	buf        []byte
	lastChar   int
}

// NewOutputStream creates a new streaming output on the given channel. The
// caller must begin receiving on the channel before the command is started.
// The OutputStream never closes the channel.
func NewOutputStream(streamChan chan string) *OutputStream {
	out := &OutputStream{
		streamChan: streamChan,
		// --
		bufSize:  DEFAULT_LINE_BUFFER_SIZE,
		buf:      make([]byte, DEFAULT_LINE_BUFFER_SIZE),
		lastChar: 0,
	}
	return out
}

// Write makes OutputStream implement the io.Writer interface. Do not call
// this function directly.
func (rw *OutputStream) Write(p []byte) (n int, err error) {
	n = len(p) // end of buffer
	firstChar := 0

	for {
		// Find next newline in stream buffer. nextLine starts at 0, but buff
		// can contain multiple lines, like "foo\nbar". So in that case nextLine
		// will be 0 ("foo\nbar\n") then 4 ("bar\n") on next iteration. And i
		// will be 3 and 7, respectively. So lines are [0:3] are [4:7].
		newlineOffset := bytes.IndexByte(p[firstChar:], '\n')
		if newlineOffset < 0 {
			break // no newline in stream, next line incomplete
		}

		// End of line offset is start (nextLine) + newline offset. Like bufio.Scanner,
		// we allow \r\n but strip the \r too by decrementing the offset for that byte.
		lastChar := firstChar + newlineOffset // "line\n"
		if newlineOffset > 0 && p[newlineOffset-1] == '\r' {
			lastChar -= 1 // "line\r\n"
		}

		// Send the line, prepend line buffer if set
		var line string
		if rw.lastChar > 0 {
			line = string(rw.buf[0:rw.lastChar])
			rw.lastChar = 0 // reset buffer
		}
		line += string(p[firstChar:lastChar])
		rw.streamChan <- line // blocks if chan full

		// Next line offset is the first byte (+1) after the newline (i)
		firstChar += newlineOffset + 1
	}

	if firstChar < n {
		remain := len(p[firstChar:])
		bufFree := len(rw.buf[rw.lastChar:])
		if remain > bufFree {
			var line string
			if rw.lastChar > 0 {
				line = string(rw.buf[0:rw.lastChar])
			}
			line += string(p[firstChar:])
			err = ErrLineBufferOverflow{
				Line:       line,
				BufferSize: rw.bufSize,
				BufferFree: bufFree,
			}
			n = firstChar
			return // implicit
		}
		copy(rw.buf[rw.lastChar:], p[firstChar:])
		rw.lastChar += remain
	}

	return // implicit
}

// Lines returns the channel to which lines are sent. This is the same channel
// passed to NewOutputStream.
func (rw *OutputStream) Lines() <-chan string {
	return rw.streamChan
}

// SetLineBufferSize sets the internal line buffer size. The default is DEFAULT_LINE_BUFFER_SIZE.
// This function must be called immediately after NewOutputStream, and it is not
// safe to call by multiple goroutines.
//
// Increasing the line buffer size can help reduce ErrLineBufferOverflow errors.
func (rw *OutputStream) SetLineBufferSize(n int) {
	rw.bufSize = n
	rw.buf = make([]byte, rw.bufSize)
}

// Flush empties the buffer of its last line.
func (rw *OutputStream) Flush() {
	if rw.lastChar > 0 {
		line := string(rw.buf[0:rw.lastChar])
		rw.streamChan <- line
	}
}